1. Field of the Invention
The present invention relates generally to fuel cell systems and, more specifically, to managing and controlling certain operating characteristics of individual fuel cells, fuel cell stacks and fuel cell arrays.
2. Background Information
Fuel cells are devices in which an electrochemical reaction is used to generate electricity. A variety of materials may be suited for use as a fuel depending upon the materials chosen for the components of the cell. Organic materials, such as methanol or natural gas, are attractive fuel choices due to the their high specific energy.
Fuel cell systems may be divided into “reformer-based” systems (i.e., those in which the fuel is processed in some fashion to extract hydrogen from the fuel before it is introduced into the fuel cell system) or “direct oxidation” systems in which the fuel is fed directly into the cell without the need for separate internal or external processing. Most currently available fuel cells are reformer-based fuel cell systems. However, because fuel processing is expensive and generally requires expensive components, which occupy significant volume, reformer based systems are presently limited to comparatively large, high power applications.
Direct oxidation fuel cell systems may be better suited for a number of applications in smaller mobile devices (e.g., mobile phones, handheld and laptop computers), as well as in some larger applications. In direct oxidation fuel cells of interest here, a carbonaceous liquid fuel (typically methanol or an aqueous methanol solution) is introduced to the anode face of a membrane electrode assembly (MEA).
One example of a direct oxidation fuel cell system is a direct methanol fuel cell system or DMFC system. In a DMFC system, a mixture comprised predominantly of methanol, or methanol and water, is used as fuel (the “fuel mixture”), and oxygen, preferably from ambient air, is used as the oxidizing agent. The fundamental reactions are the anodic oxidation of the fuel mixture into CO2, protons, and electrons; and the cathodic combination of protons, electrons and oxygen into water. The overall reaction may be limited by the failure of either of these reactions to proceed to completion at an acceptable rate, as is discussed further hereinafter.
Typical DMFC systems include a fuel source, fluid and effluent management systems, and air management systems, as well as a direct methanol fuel cell (“fuel cell”). The fuel cell typically consists of a housing, hardware for current collection and fuel and air distribution, and a membrane electrode assembly (“MEA”) disposed within the housing.
The electricity generating reactions and the current collection in a direct oxidation fuel cell system generally take place within the MEA. In the fuel oxidation process at the anode, the products are protons, electrons and carbon dioxide. Protons (from hydrogen found in the fuel and water molecules involved in the anodic reaction) are separated from the electrons. The protons migrate through the membrane electrolyte, which is impermeable to the electrons. The electrons travel through an external circuit, which connects the load, and are united with the protons and oxygen molecules in the cathodic reaction, thus providing electrical power from the fuel cell.
A typical MEA includes a centrally disposed protonically-conductive, electronically non-conductive membrane (“PCM”, sometimes also referred to herein as “the catalyzed membrane”). One example of a commercially available PCM is NAFION® a registered trademark of E.I. Dupont de Nemours and Company, a cation exchange membrane based on polyperflourosulfonic acid, in a variety of thicknesses and equivalent weights. The PCM is typically coated on each face with an electrocatalyst such as platinum, or platinum/ruthenium mixtures or alloy particles. On either face of the catalyst coated PCM, the electrode assembly typically includes a diffusion layer. The diffusion layer on the anode side is employed to evenly distribute the liquid fuel mixture across the catalyzed anode face of the PCM, while allowing the gaseous product of the reaction, typically carbon dioxide, to move away from the anode face of the PCM. In the case of the cathode side, a diffusion layer is used to allow a sufficient supply of and a more uniform distribution of gaseous oxygen across the cathode face of the PCM, while minimizing or eliminating the collection of liquid, typically water, on the cathode aspect of the PCM. Each of the anode and cathode diffusion layers also assist in the collection and conduction of electric current from the catalyzed PCM through the load.
Direct oxidation fuel cell systems for portable electronic devices should be as small as possible at the power output required. The power output is governed by the rate of the reactions that occur at the anode and the cathode of the fuel cell. More specifically, the anode process in direct methanol fuel cells based on acidic electrolytes, including polyperflourosulfonic acid and similar polymer electrolytes, involves a reaction of one molecule of methanol with one molecule of water. In this process, the oxygen atom in the water molecule is electrochemically activated to complete the oxidation of methanol to a final CO2 product in a six-electron process.
More specifically, direct methanol fuel cell system produces electricity without combustion by oxidizing a carbonaceous fuel (typically methanol in an aqueous solution) on a catalyzed protonically conductive membrane.                The electrochemical reaction equations are as follows:Anode: CH3OH+H2O=CO2+6H++6e−  Equation 1Cathode: 6H++6e−+3/2O2=3H2O   Equation 2Net Process: CH3OH+3/2O2═CO2+2H2O   Equation 3        
Generation of electricity continues until one of the fluids is not available. DMFCs are typically described as “on” i.e. providing electrical current by reacting the fuel and oxygen to generate water, or “off” i.e. at least one fluid is not available because all fuel has been consumed, or air (or other source of oxygen) is prevented from reaching the cathode face of the PCM. Those skilled in the art will recognize that fuel can be delivered to the anode aspect of the MEA as a liquid, or in vaporous form.
Thus, the efficiency of a direct methanol fuel cell system is dependent in part on the amount of fluids and products that are present in the active catalyzed membrane areas and also depends upon adequate hydration of the membrane. For example, particularly in a vapor fed cell, there is a tendency for the catalyzed membrane to dry out during operation, or when the fuel cell is shut down. This is because the vapor feed is not aqueous, instead it is substantially pure fuel, such that there is essentially no excess water on the anode side to keep the membrane hydrated. However, as stated, the membrane should remain well-hydrated for optimal performance. Other operating parameters and other conditions are also important to monitor, in order to maintain the desired fuel cell performance.
More specifically, operating parameters of individual fuel cells in a stack, for example, should optimally be regulated and checked. More specifically, for a given architecture, an optimal voltage exists at which efficiency is maximized at a given fuel cell concentration. For example, at 1.5M (molar) fuel, some cells produce the best overall efficiency when loaded to the point that they produce 0.3 V (volts). Bipolar stacks, for example, consist of many DMFCs in series. In a seven cell stack, with all cells operating optimally, it would be expected that the optimal stack voltage would be about 2.1V. Yet, different cells perform at different levels due to physical manufacturing variations and, more importantly, to anode fuel delivery and CO2 removal, as well as cathode O2 delivery and water removal. For this reason, while driving the overall stack to 2.1V forces the average cell voltage to 0.3V by definition, it does not force all cells to that exact amount, instead there is a minimum and maximum individual cell output voltage. In fact, in certain scenarios, cells can become reversed and do negative work or in extreme prolonged cases can become damaged. Thus, it would be desirable to be able to check each cell and maintain each cell at a specified voltage. However, up to now it has not been straightforward to perform checks that would maximize efficiency and correct asymmetries by keeping all of the cells at an optimal voltage.
Another operating parameter that is desirable to control is the output voltage of the overall stack or array. The optimal stack output voltage value depends upon the mode of operation of the fuel cell system. For example, during normal operations, there is a “RUN” voltage at which the stack should be operating such that the cells in the stack are being run at a normal rate for a given fuel cell concentration. There are other situations in which the stack may be operated as a “hot stack.” It is also desirable to maintain a maximum stack current thus requiring that the stack current not drop below that amount.
Furthermore, a fuel cell may be contained within a power supply unit that also contains an internal battery. The power supply unit may then be used to power an application device that itself includes a rechargeable battery (the “external battery”). It may be desirable to cause the internal battery to operate at a particular current level and not drop below that while the external battery is charging. There are also limits on the amount that the external batteries can be charged, in that it is undesirable to overcharge the internal battery, or the external battery. Thus, output current is important in battery charging and management.
Power requirements are also important. As will be understood by those skilled in the art, it may be inefficient to run a fuel cell system at a higher power than required by the application device to which it is delivering power. Thus, a maximum power may be a parameter that is desired to be controlled. Temperature and concentration are also important operating characteristics to be considered in fuel cell design and operation.
It has not always been straightforward, however, to obtain a reading or measurement of various parameters existing at one particular time in a fuel cell or a fuel cell system, given the number of microcomponents in a microfuel cell. In addition, fuel cell operating conditions can change randomly based upon user input. For example, if the fuel cell is powering a wireless telephone, for example, the user may press the “send” button or turn the power on or off. These actions by the user are unpredictable yet they affect the underlying operation of the fuel cell system. Thus, the system ideally should be in a position to dynamically react to such changes in power supply needs or operating conditions.
It has been known to provide a method and apparatus for controlling the operating point, i.e. the output voltage or current of a fuel cell, to a desired value such as that described in commonly-owned U.S. Pat. No. 6,590,370, issued Jul. 8, 2003 of Leach, for a SWITCHING DC-DC POWER CONVERTER AND BATTERY CHARGER FOR USE WITH DIRECT OXIDATION FUEL CELL POWER SOURCE, which is incorporated herein by reference.
However, up to now it has not been practical to dynamically take measurements required to evaluate the above-described operating conditions and parameters. And, a convenient and readily available means for taking corrective action has not been known when it is indicated that such actions are needed based upon the measurements taken. Some measurement devices and corrective features that are known involve large expensive equipment, suitable only for use in a laboratory setting. It is desirable to have an on-board diagnostics and a control system that performs the measurements and takes corrective actions and which is amenable to use within a consumer electronic device.
Therefore, there remains a need for a more easily implemented method and apparatus for measuring operating parameters in a direct oxidation fuel cell, a direct oxidation fuel cell stack or a direct oxidation fuel cell array and for taking corrective actions based on such measurements.
It is thus an object of the invention to provide a method and apparatus for readily measuring various operating parameters of the fuel cell, the fuel cell stack or the fuel cell array, which can provide information and, more particularly, can signal corrective action to be taken based upon such information.